Since the advent of television, good horizontal line synchronization characteristics have been a most crucial performance parameter of television (TV) receivers and video decoders. A key parameter of the performance of a video decoder is determined by its synchronization properties. Even under worst case circumstances, an output image generated by the video decoder must be horizontally and vertically aligned with the original image as captured by the signal source. Thus, elaborate horizontal synchronization schemes and systems have evolved, aiming to achieve robust synchronization particularly in the presence of noise—a noisy signal condition. This condition occurs due to the weak amplitude of the RF signal input to the TV tuner.
With the market introduction of helical scan video recorders, such as VHS and Beta, a new class of synchronization requirements arose, where the time base stability, both horizontal and vertical scanning, may be impaired by timing fluctuations. The demand for good noisy signal performance requires a slow reaction, i.e. a long time constant combined with minimal damping of the synchronization phase locked loop (PLL); while, time base instabilities of video recorders, on the other hand, require fast action, i.e. a short time constant combined with strong damping. Thus, both scenarios are contradictory as they require opposing synchronization strategies for optimum results. Consequently, non-adaptive implementations of the synchronization functions suffer insufficient performance for either of the two worst case scenarios.
In order not to compromise performance, most TV receivers deal with this problem by means of two distinctly different sets of characteristics of their horizontal phase locked loop (PLL) as is disclosed in “Television Engineering Handbook,” K. Blair Benson & Jerry C. Whitaker, McGraw-Hill Inc., pp. 13.163-13.166, which is incorporated by reference herein. As is described, for various operating conditions, a choice of parameters proves to be nearly optimum: noisy-signal performance, natural frequency, fnn=500 Hz, damping factor, K˜1; video-cassette recorder operation, fnn=1000 Hz, K˜18.
Often the viewer has to choose the appropriate mode whether television or VCR which leaves room for misinterpretation. Several attempts, however, have been made for automatic identification and adaptation of the appropriate mode as is also disclosed in “Television Engineering Handbook,” K. Blair Benson & Jerry C. Whitaker, McGraw-Hill Inc., chapter 13.14.3. In U.S. Pat. No. 5,953,071, (col. 2 line. 11-33), which is incorporated by reference herein, its objective is to automatically adapt to the characteristics of a particular signal source and more particularly to auto-adaptively change the bandwidth of the loop filter.
Generally, these aforementioned approaches use a noise estimate measurement to control the loop gain of the horizontal synchronization PLL to approximate an optimum match of the PLL characteristics. Other approaches, such as, coincidence detection, dual-loop PLL usage, adaptive loop control, and behavioral enhancement during the vertical blanking interval, are disclosed in “Synchronization Issues for Consumer TV,” Kees van Zon, Proceedings 1995, IEEE International Symposium on Synchronization, Dec. 14-15, 1995 pp.165-170, which is incorporated by reference herein.
There, however, exists a need for a video decoder having the capability of distinguishing between a television and VCR signal and, furthermore, distinguishing between a noisy signal and normal signal; and thereby, applying the appropriate optimum signal processing technique to the signal.